In any high-speed data link, it can be challenging to recover correctly at the receiving side data that have been transmitted. With ever-increasing bandwidth, high frequency transmission may incur more loss in the channel that is connecting receiver to transmitter.
To ensure signal integrity at both transmitting side and receiving side, some conventional systems include a transmitter with a certain output impedance to match the characteristic impedance of the channel. At the receiving side, there is a receiver with termination impedance typically matching the characteristic impedance of the channel. This is done with signal integrity as a goal because impedance matching reduces or eliminates reflected energy from the receiving side. Moreover, any reflected energy from the receiver will undergo secondary reflections back at the transmitter should transmitting impedance not match the channel impedance, thereby further degrading signal integrity of subsequent bits at the receiver.
Conventional transmission lines in high-speed links may have a characteristic impedance of about 50 ohms However, setting the receiver termination resistance to as low as 50 ohms may lead to undesirably high power consumption incurred by the transmitter. But simply increasing the termination resistance at the receiver to reduce power consumption may not be a proper solution because it would increase reflected energy, thereby reducing signal integrity that could ultimately result in bit errors at the receiver. Accordingly, power consumption and signal integrity compete for consideration in conventional designs.
Furthermore, conventional Double Data Rate (DDR) memory interfaces may deviate sometimes from a matched impedance. For instance, as signal bit rates increase, a conventional DDR system may adjust a termination resistance at the receiver to match or nearly match the impedance of the channel. But as signal bit rates are decreased, it may be possible to increase receiver termination resistance in order to save power as long as the signal is still recoverable. However, for a given bit rate, there is a limit to the amount of impedance mismatch that may be tolerated.
It would be desirable to use a higher impedance with higher bandwidths while maintaining integrity of the received signal.